I have a similar issue, my procaster will not pick up the audio from my computer. It will apparently pick up my mic fine, but not game audio or from my desktop.
All my viewers hear is static.
I've tried editing the sources in the audio mixer, but there is no "speaker audio" option.
Only what is shown here: http://i.imgur.com/huL2D.png
I have realtek for my sound, and choosing the realtek option does nothing.
My OS is Win7.